Microstructural modification of melt spun iron alloys via high shear deformation to enhance rare-earth free magnets

Abstract

Rare-earth hard magnets, such as NdFeB, have seen increasing demand and subsequently increasing price in recent years. Thus, seeking alternatives to rare-earth containing magnets has become increasingly important. Iron-nitrogen magnets have been of interest because of their high magnetic properties, including remanence, coercivity, and temperature characteristics. However, creating a bulk magnet out of the iron and nitrogen has been difficult. In this research, melt spun ribbons of iron alloys were studied to determine the feasibility of microstructural modification through high shear deformation processing prior to nitriding. Specifically, this study was to determine if grain refinement, formation of high angle grain boundaries, and microstructure homogenization could be achieved through high shear deformation processing. Three generations of high shear deformation processing instruments were utilized to adequately process different sizes of pure iron strips and melt spun iron ribbons. These generations include a modified rolling mill, a linear fretting tribometer, and a compression frame. High shear deformation processing was completed on pure iron strips initially to prove that high shear deformation can homogenize melt spun microstructures, refine grain size, and create high angle grain boundaries. These microstructural modifications can aid in nitriding and ultimately increase the coercivity and maximum energy product. After successful processing of thick iron strips, processing was completed on the melt spun FeNbB ribbons. The melt spun FeNbB ribbons were successfully processed to homogenize as-solidified microstructures, refine grains, and to create both new grain boundaries and high angle grain boundaries, regardless of the initial microstructure of the melt spun ribbons.